Major challenges remain in controlled administration of insoluble and toxic hydrophobic drugs to target sites. Although tremendous progress has been made in the field of delivery vehicle design, the critical issues of encapsulation stability and versatility of the delivery vehicles continue to present major difficulties. (Goldberg, et al. 2007 J. Biomater. Sci.—Polym. E. 18, 241-268; Allen, et al. 2004 Science 303, 1818-1822; Savic, et al. 2006 J. Drug. Target 14, 343-355.) A goal that continues to elude researchers is a functional system wherein a water-soluble container non-covalently binds hydrophobic guest molecules and releases them in a controlled manner in response to a specific trigger. (Peer, et al. 2007 Nat. Nanotechnol. 2, 751-760; Haag 2004 Angew. Chem. Int. Ed. 43, 278-282; Ganta, et al. 2008 J. Control. Release 126, 187-204; Allen, et al. 2004 Science 303, 1818-1822.) When such a container is based on a nano-sized host, there is an even greater interest because of the potential in passive targeting of tumor tissue through the so-called enhanced permeability and retention (EPR) effect. (Maeda, et al. 2000 J. Control. Release 65, 271-284.)
Nano-scale supramolecular micellar assemblies are promising candidates because they are capable of non-covalently sequestering hydrophobic guest molecules in aqueous solution. (Duncan 2003 Nat. Rev. Drug Discovery 2, 347-360; Liu, et al. 2009 Macromolecules 42, 3-13; Davis, et al. 2008 Nat. Rev. Drug Discovery 7, 771-782; Kataoka, et al. 2001 Adv. Drug Deliv. Rev. 47, 113-131; Savić, et al. 2003 Science 300, 615-618.) However, micellar assemblies formed from small molecule surfactants have inherent stability issues.
Water-soluble cross-linked polymer nanoparticles or nanogels that can sequester lipophilic guest molecules within their interiors is of great interest in various applications ranging from delivery vehicles for therapeutics, to diagnostics to theranostics, among others. (Farokhzad, et al. 2004 Science 303, 1818.) However, the classical preparative methods including microemulsion or inverse microemulsion ones do not conveniently allow the nanogels to be water-soluble and encapsulate lipophilic guest molecules simultaneously. (Bachelder, et al. 2008 J. Am. Chem. Soc. 130, 10494; Oh, et al. 2007 J. Am. Chem. Soc. 129, 5939.)
Assemblies formed from amphiphilic polymers tend to exhibit enhanced stabilities, although they face significant complications because of a requisite concentration for assembly formation, which drastically limits the practicality of in vivo micelle utilization. Large dilution of injected micelles into the body can destabilize the self-assembling systems and cause uncontrolled and undesirable release of the encapsulated drug payload before arrival at the target site. (Bae, et al. 2008 J. Control. Release 131, 2-4.) Moreover, the interaction between micelles and biological components, such as cellular membranes and blood components, can lead to premature release of the payload from the micelle core. (Chen, et al. 2008 Proc. Natl. Acad. Sci. U.S.A. 105, 6596-6601; Chen, et al. 2008 Langmuir 24, 5213-5217.) Therefore, alternate strategies are desired to overcome such premature release.
Due to their cross-linked nature, polymer nanogels potentially provide both high encapsulation stability and potential for triggered release. (Byrne, et al. 2002 Adv. Drug Deliv. Rev. 54, 149-161; Kabanov, et al. 2009 Angew. Chem. Int. Ed. 48, 5418-5429; Kopecek 2002 Nature 417, 388-391; Hamidi, et al. 2008 Adv. Drug Deliv. Rev. 60, 1638-1649.) Current synthetic methods for nanogel preparation are based on water-in-oil emulsion, in which inverse micelles, formed from surfactants in non polar solvent, provide an aqueous interior as a reaction template for polymerization. (Bachelder, et al. 2008 J. Am. Chem. Soc. 130, 10494-10495; Sission, et al. 2009 Angew. Chem. Int. Ed. 48, 7540-7545; Kriwet, et al. 1998 J. Controlled Release 56, 149-158; Oh, et al. 2008 Prog. Polym. Sci. 33, 448-477.)
The reported nanoparticles or nanogels, however, suffer from significant limitations as they are prepared by microemulsion or inverse microemulsion methods. (Sisson, et al. 2009 Angew. Chem. Int. Ed. 48, 7540-7545; Bachelder, et al. 2008 J. Am. Chem. Soc. 130, 10494-10495; Oh, et al. 2007 J. Am. Chem. Soc. 129, 5939-5945.) These methods are complex and require multiple purification steps to remove unreacted monomers and surfactant materials that were used to stabilize the emulsion. When a water-soluble polymer nanoparticle is targeted, inverse microemulsion-based synthesis is a preferred method. The continuous phase in the inverse microemulsion (water-in-oil emulsion) method is based on a lipophilic solvent and, therefore, cannot be used to encapsulate hydrophobic guest molecules during nanoparticle formation. An attractive alternate to forming polymer nanoparticles is to collapse a limited number of polymer chains to achieve the desired nanoparticles. The reported methods required ultrahigh dilution conditions or inverse addition conditions, which significantly limit the capabilities in guest molecule incorporation. (Kadlubowski, et al. 2003 Macromolecules 36, 2484-2492; Jiang, et al. 2005 Macromolecules 38, 5886-5891; Mackay, et al. 2003 Nat. Mater. 2, 762-766; Cheman, et al. 2007 J. Am. Chem. Soc. 129, 11350-11351; Harth, et al. 2002 J. Am. Chem. Soc. 124, 8653-8660.)
Furthermore, nanoscale vehicles are desired that can concurrently sequester and deliver two different molecules. (Kelkar, et al. 2011 Bioconjug. Chem. 22, 1879-1903; Kelkar, et al. 2011 Acc. Chem. Res. 44, Issue #10; Jain 2001 Nature Med. 7, 987-989; Sengupta, et al. 2005 Nature, 436, 568-572.) It is difficult and complicated when a combination of a water-soluble hydrophilic molecule and a water-insoluble lipophilic molecule are to be co-encapsulated and delivered, partly due to the tendency of proteins to irreversibly denature under non-native conditions. (Kim, et al. 2009 Langmuir 25, 14086-14092; Kim, et al. 2011 Mol. Pharmaceutics. 8, 1955-1961; Wiradharma, et al. 2009 Biomaterials 30, 3100-3109.)
Targeting ligands, such as folic acid, have been studied as components of active targeting systems in drug delivery. (Nasongkla, et al. 2004 Angew. Chem. Int. Ed. 43, 6323-6327; Lee, et al. 2008 Angew. Chem. Int. Ed., 47, 2418-2421; Aluri, et al. 2009 Adv. Drug Delivery Rev., 61, 940-952; Sudimack, et al. 2000 Adv. Drug Delivery. Rev. 41, 147-161.) Ligands are attached to the hydrophilic ends of assembly-forming amphiphilic molecules. (Sutton, et al. 2007 Pharm. Res. 24, 1029-1046; Yoo, et al. 2004 J. Controlled Release 96, 273-283; Xu, et al. 2007 Angew. Chem. Int. Ed. 46, 4999-5002.) However, the installation of such ligands onto these molecules requires complicated synthetic steps, limiting the versatility of ligand functionalization to tailor carriers for targeting specific cell types.
Thus, novel and improved polymer nano-assemblies and methods of preparation are required to overcome the limitations and shortcomings of the existing methods, in particular, those that can transport diverse payloads and with versatile and effective targeting capabilities are strongly desired.